The smoke unit in question is a 6 Ohm model that Lionel stopped using several years ago - it was a prolific smoker used in early Legacy engines like the FEF-3 and GS-4 but prone to blow the AC regulator. Lionel later recommended substituting an 8 Ohm resistor. I recall that for some length of time after that, a replacement PCB as opposed to the entire smoke unit assembly was available but unfortunately I no longer see that in Lionel's replacement parts pages.
I am not sure that there is any engine-specific logic on the PCB that would prevent substituting another smoke unit if you have to - hopefully someone more expert can opine on that. However, I have worked on a number of these and the PCB itself seldom fails but a component on it may have gone. I can't guess which one it might be because don't quite follow where you are measuring the power to the board and resistor and getting a track voltage reading. I suppose it's possible that only the resistor has gone and you might want to open the unit up to have a look at it.
